1. Field of the Invention
The invention relates to nacelles for aircraft engines, and more particularly relates to an improved nacelle for a turbofan engine having an inlet cowl that is designed to assist in achieving a stable fly-home configuration subsequent to a blade-out event.
2. Description of the Related Art
A nacelle for a turbofan engine must meet several basic design criteria. For example, the nacelle should direct air flow to the air intake of the engine while protecting the air flow from disturbances such as gusts, and the like. In addition, the exterior surface profile of the nacelle should minimize the aerodynamic drag caused by the engine and its related components.
As shown in FIGS. 1A and 1B, a modern turbofan engine assembly 10 typically includes a nacelle 22 and a fan case 16. The engine assembly, including the nacelle 22 and fan case 16, can be suspended from an aircraft's wing by a pylon 12. In FIG. 1A, one side of the nacelle structure 22 is removed for ease of illustration. The fan case 16 surrounds the engine's fan 18. The fan 18 includes a plurality of fan blades 19 attached to the engine's rotor. As shown in FIG. 1A, a typical nacelle structure 22 includes a forward inlet portion 24 and an aft nacelle portion 25. The inlet portion 24 is typically attached to a forward flange 14 on the fan case 16 by a plurality of circumferentially spaced fasteners, such as bolts or the like. As shown in FIGS. 1A and 1B, the inlet portion 24 typically includes an outer barrel 32, a rounded nose lip section 28, an inner barrel 30, and one or more spaced bulkheads 34, 36 disposed between the outer barrel 32 and the inner barrel portion 30. The outer barrel portion 32 and nose lip portion 28 can be constructed of a thin metallic material, such as aluminum, for example, or can be constructed of composite materials. The inner barrel 30 typically is constructed of composite materials and includes acoustic treatment configured to attenuate at least some engine noise. Such an acoustically treated inner barrel 30 typically includes a honeycomb core 31 sandwiched between a perforated composite inner skin 33 and an imperforate composite outer skin 35. The composite inner barrel 30 can be constructed in two or more circumferential segments joined together by fasteners, or can be an unsegmented, one-piece composite structure. Some advantages of a one-piece inner barrel 30 over a segmented inner barrel 30 include fewer parts and fasteners, a seamless aerodynamic inner surface, and lower manufacturing costs, for example.
A forward edge 39 of the outer barrel 32 can be connected to the nose lip portion 28 by a first plurality of circumferentially spaced fasteners 47, such as rivets, or the like. Similarly, a forward edge of inner barrel 30 can be connected to the nose lip portion 28 by a second plurality of circumferentially spaced fasteners 37, such as rivets, bolts, or the like. The fasteners 37, 47 secure the components of the inlet portion 24 together, and transmit loads between fastened components. In the embodiment shown in FIG. 1B, a forward bulkhead 38 extends between the outer and inner walls of the nose lip 28, and an intermediate bulkhead 34 and an aft bulkhead 36 connect portions of the outer barrel 32 and the inner barrel 30. The bulkheads 34, 36 contribute to the rigidity and strength of the inlet portion 24. In addition, the intermediate and aft bulkheads 34, 36 transmit loads between the inner barrel 30 and the outer barrel 32. As shown in FIG. 1B, an aft flange 41 on the inner barrel 30 can connect the inlet portion 24 to a forward flange 14 of a fan case 16. Accordingly, the composite inner barrel 30 directly supports the outer barrel 32 and nose lip portion 28. The weight of the inlet portion 24 and external loads borne by the inlet portion 24 are necessarily transferred to the fan case 16 through the inner barrel 30. Therefore, the composite inner barrel 30 of a typical nacelle inlet 24 can substantially contribute to the overall rigidity, strength and stability of the inlet portion 24 of the nacelle 22.
The bulkheads 34, 36, 38 shown in FIG. 1B typically are constructed of a thin metallic material such as aluminum, for example. The bulkheads 34, 36, 38 can be welded to the metallic outer barrel 32 and metallic nose lip portion 28, or can be connected to the outer barrel 32 and/or nose lip portion 28 by mechanical fasteners, such as rivets, or the like. The aft bulkhead 36 and intermediate bulkhead 34 can be fastened to the composite inner barrel 30 by mechanical fasteners such as rivets, bolts, or the like. As shown in FIG. 1B, one or more circumferentially extending reinforcement ribs 21 can be welded or otherwise attached along the inner surface of the outer barrel 32 to stiffen the thin metal skin and maintain an aerodynamic shape.
As discussed below, a typical nacelle structure like that shown in FIGS. 1A and 1B and described above can be improved. U.S. Federal Aviation Administration (FAA) regulations set forth numerous design objectives for aircraft. For example, the structural integrity of an aircraft engine nacelle should be sufficient to permit an associated aircraft to be safely flown and landed following a blade-out event. More specifically, a nacelle 22 should maintain a stable and aerodynamic configuration that will not impede the fly-home capability of an aircraft following a blade-out event. As is known in the art, a “blade out event” arises when a blade is accidentally released from a turbine's rotor, such as when a first-stage fan blade 19 is accidentally released from a rotor of a high-bypass turbofan engine 10. When suddenly released during flight, a fan blade 19 can impact a surrounding fan case 16 with substantial force, and resulting loads on the fan case 16 can be transferred to surrounding structures, such as to the inlet portion 24 of a surrounding nacelle 22. These loads can cause substantial damage to the nacelle inlet 24, including damage to an adjoined inner barrel 30. In addition or alternatively, a released fan blade 19 can directly impact a portion of an adjacent inner barrel 30, thereby causing direct damage to the inner barrel 30. Because the inner barrel 30 directly supports the inlet portion 24 on the fan case 16, including the outer barrel 32 and nose lip portion 28, damage to the inner barrel 30 can compromise the structural integrity and stability of the nacelle inlet 24, and may negatively affect the fly-home capability of an aircraft.
A blade-out event also causes the rotational balance of an engine's fan 18 to be lost. After a damaged engine 10 is shut down following a blade-out event, airflow impinging on the unbalanced fan 18 can cause the fan 18 to rapidly spin or “windmill.” Such wind-milling of an unbalanced fan 18 can exert substantial vibrational loads on the engine 10 and fan case 16, and at least some of these loads can be transmitted to an attached inlet portion 24 and inner barrel 30 of the nacelle 22. In addition, following a blade-out event, aerodynamic forces and a suction created by a windmilling fan 18 can exert substantial loads on a damaged inlet portion 24 of the nacelle 22. Such loads can cause substantial deformation of a damaged inlet portion 24 and can result in unwanted aerodynamic drag.
Such loads also can cause cracks or breaks in a damaged composite inner barrel 30 to propagate, further compromising the structural integrity and stability of a damaged inlet portion 24 of a nacelle 22. Without crack-stopping longitudinal joints or reinforced flanges between adjoined circumferential segments of an inner barrel 30, such crack propagation can be more severe in a one-piece inner barrel than in a segmented inner barrel.
As discussed above, the inner barrel 30 of a typical nacelle inlet 24 substantially contributes to the overall strength and rigidity of nacelle inlet's structure. Accordingly, when the inner barrel 30 of an inlet portion 24 of a nacelle is substantially damaged subsequent to a blade-out event, the structural integrity and rigidity of a nacelle's inlet portion 24 may not be sufficient to adequately withstand such suction and/or aerodynamic loads, or to maintain a stable and aerodynamic configuration of the nacelle inlet 24 that is sufficient to support the fly-home capability of an aircraft.
Accordingly, there is a need for a nacelle structure for a turbofan aircraft engine that is capable of maintaining a substantially stable and aerodynamic configuration subsequent to a blade-out event, and which thereby supports an aircraft's fly home capability following such an incident. In particular, there is a need for a nacelle inlet structure for a high-bypass turbofan aircraft engine that maintains its structural integrity and a stable aerodynamic configuration even though its composite inner barrel has been substantially damaged due to a blade-out event. Preferably such an improved nacelle inlet will include a minimal number of components in order to minimize weight and minimize manufacturing costs.